1. Field of the Invention
The present invention relates generally to expandable medical implants for supporting a body lumen and, more particularly, to expandable, intraluminal devices, generally referred to as stents.
2. Description of the Related Art
Stents are implanted into body lumens, such as blood vessels, to maintain the patency of the lumens. These devices are frequently used in the treatment of atherosclerotic stenoses in blood vessels, especially in conjunction with percutaneous transluminal coronary angioplasty (PTCA) procedures. After treating a blood vessel, a stent is implanted to support the vessel wall and thereby reduce the likelihood of restenosis. Stents are most commonly implanted in coronary arteries; however, stents may also be used in a wide variety of other body lumens. For example, stents may be deployed in the biliary, carotid, superficial femoral and popliteal arteries or even veins
Over the years, a wide variety of stent types have been proposed. Although the structures of stents may vary substantially, virtually all stents are configured to be expandable from a collapsed condition having a small diameter to an expanded condition having a larger diameter. While in the collapsed condition, the stent is delivered through the blood vessel, or other body lumen, to the treatment site. After the treatment site is reached, the stent is radially expanded to an implantable size for supporting the vessel wall. Expansion of the stent from the collapsed condition to the expanded condition can be achieved in a variety of different ways. Various types of stents are described below based on their means for expansion. For additional information, a variety of stents types are described by Balcon et al., “Recommendations on Stent Manufacture, Implantation and Utilization,” European Heart Journal (1997), vol. 18, pages 1536-1547, and Phillips, et al., “The Stenter's Notebook,” Physician's Press (1998), Birmingham, Mich.
Balloon expandable stents are manufactured in the collapsed condition and are expanded to a desired diameter with a balloon. During delivery, a balloon expandable stent is typically mounted on the exterior of an inflatable balloon located along the distal end portion of a catheter. After reaching the treatment site, the stent is expanded from the collapsed condition to the expanded condition by inflating the balloon. The stent is typically expanded to a diameter that is greater than or equal to the inner diameter of the body lumen. The expandable stent structure may be held in the expanded condition by mechanical deformation of the stent as taught in, for example, U.S. Pat. No. 4,733,665 to Palmaz. Alternatively, balloon expandable stents may be held in the expanded condition by engagement of the stent walls with respect to one another as disclosed in, for example, U.S. Pat. Nos. 4,740,207 to Kreamer, 4,877,030 to Beck et al., and 5,007,926 to Derbyshire. Further still, the stent may be held in the expanded condition by one-way engagement of the stent walls together with endothelial growth into the stent, as disclosed in U.S. Pat. No. 5,059,211 to Stack et al.
Balloon expandable stents are typically manufactured from stainless steel and generally have a high radial strength. The term “radial strength,” as used herein, describes the external pressure that a stent is able to withstand without incurring clinically significant damage. Due to their high radial strength, balloon expandable stents are commonly used in the coronary arteries to ensure patency of the vessel. During deployment in a body lumen, the inflation of the balloon can be regulated for expanding the stent to a particular desired diameter. Accordingly, balloon expandable stents are often preferred in applications wherein precise placement and sizing are important. Balloon expandable stents are also commonly used for direct stenting, wherein there is no pre-dilation of the vessel before stent deployment. Rather, during direct stenting, the expansion of the inflatable balloon dilates the vessel while also expanding the stent.
Although balloon expandable stents are the first stent type to be widely used in clinical applications, it is well recognized that balloon expandable stents suffer from a variety of shortcomings which may limit their effectiveness in many important applications. For example, in one significant shortcoming, existing balloon expandable stents are not biased toward the expanded condition and therefore do not return to the expanded condition after being deformed, bent, or pinched. Accordingly, when a high external pressure overcomes the radial strength of a balloon expandable stent, the stent may be caused to permanently deform inward (i.e. collapse) such that the lumen is substantially reduced in size. Worse yet, external pressures may cause the stent to completely collapse, with potentially fatal clinical implications. Therefore, balloon expandable stents are not well-adapted for use in blood vessels which are subjected to large torsional or flexion/extension stresses (e.g., the superficial femoral artery and popliteal artery) and/or wherein the stent is vulnerable to large external pressures (e.g., the superficial femoral artery and carotid artery).
In another shortcoming, balloon expandable stents often exhibit substantial recoil (i.e., a reduction in diameter) immediately following deflation of the inflatable balloon. Accordingly, it may be necessary to over-inflate the balloon during deployment of the stent to compensate for the subsequent recoil. This is disadvantageous because it has been found that over-inflation may damage the blood vessel. Furthermore, a deployed balloon expandable stent may exhibit chronic recoil over time, thereby reducing the patency of the lumen. Still further, balloon expandable stents often exhibit foreshortening (i.e., a reduction in length) during expansion, thereby creating undesirable stresses along the vessel wall and making stent placement less precise. Still further, many balloon expandable stents, such as the original Palmaz-Schatz stent and later variations, are configured with an expandable mesh having relatively jagged terminal prongs, which increases the risk of injury to the vessel, thrombosis and/or restenosis.
Self-expanding stents are manufactured with a diameter approximately equal to, or larger than, the vessel diameter and are collapsed and constrained at a smaller diameter for delivery to the treatment site. Self-expanding stents may be placed within a sheath or sleeve to constrain the stent in the collapsed condition during delivery. Alternatively, detachable tabs or pins may be used for locking the stent in the collapsed condition. After the treatment site is reached, the constraint mechanism is removed and the stent self-expands to the expanded condition. Typically, self-expansion of the stent results from the inherent properties of the material constituting the stent. Most commonly, self-expanding stents are made of Nitinol or other shape memory alloy.
Because self-expanding stents are biased towards the preset expanded condition, if the self-expanding stent is caused to deform under pressure, the stent will return to its expanded condition when the pressure is removed. Accordingly, self-expanding stents overcome many of the shortcomings, such as the risk of permanent collapse, associated with balloon expandable stents. Therefore, self-expanding stents are often deployed in areas of the body where large external forces may cause the vessel, and therefore the stent, to temporarily deform radially inward. After the external force is reduced or removed, the self-expanding stent returns to its fully expanded condition, thereby eliminating the danger of permanent stent deformation and obstruction of the lumen.
One of the first self-expanding stents used clinically is the braided “WallStent,” as described in U.S. Pat. No. 4,954,126 to Wallsten. The WallStent generally comprises a metallic mesh in the form of a Chinese finger cuff. The cuff provides a braided stent that is not superelastic, but technically still falls in the self-expanding stent family. Although the WallStent provided a significant improvement in stent technology for certain applications, such as the treatment of long lesions, the WallStent and other stents of this type often exhibit undesirable metal prongs that remain along the longitudinal ends thereof as a result of the manufacturing process. Another disadvantage of the WallStent is the inherent rigidity of the material (e.g., a cobalt-based alloy having a platinum core) used to form the stent. The combination of the rigidity and the terminal prongs has been found to produce substantial difficulties during navigation through the patient's vasculature. Accordingly, the procedure produces undesirable risks from the standpoint of injury to healthy tissue along the passage to the target vessel.
Another example of a self-expanding stent is disclosed in U.S. Pat. No. 5,192,307 to Wall wherein a stent-like prosthesis is formed of plastic or sheet metal that is expandable or contractible for placement. The stent may be biased in an open position and lockable in a closed position or, alternatively, may be biased towards a closed position and lockable in an open position. In the former case, a pin may be used to hold the stent in the collapsed condition. The pin is removed to allow the stent to assume the expanded condition. One or more hooks may be formed into the wall for locking the stent. The hooks engage complementary recesses formed in an opposing wall to mechanically interlock the rolled up sheet forming the stent.
Although self-expanding stents provide a number of advantages over balloon expandable stents, self-expanding stents also suffer from a wide variety of shortcomings. In one well-recognized shortcoming, self-expanding stents lack the high radial strength of balloon expandable stents and therefore self-expanding stents may deform under relatively low external pressures. In another shortcoming, self-expandable stents often exhibit significant foreshortening during radial expansion. As a result, stents of this type may not provide predictable longitudinal coverage when fully deployed. Furthermore, self-expanding stents necessarily require a constraining mechanism for holding the stent in the collapsed condition during delivery. For example, as described above, a self-expanding stent may be placed in a separate deployment sheath for constraining the stent during delivery. During deployment of a self-expanding stent, the sheath is retracted to uncover the stent incrementally from the distal end to the proximal end, thereby allowing the stent to expand from one end to the other. However, this often results in the stent jumping or springing forward from the delivery system in an undesirable manner, sometimes causing the stent to buckle or bunch up during delivery. Still further, it has been found that self-expanding stents do not re-dilate well in cases of re-treatment and are not well suited for direct stenting.
In yet another shortcoming, self-expanding stents typically impose a continuous chronic outward stress on the vessel wall that may create significant risks of damage to the vessel wall and may lead to restenosis. It is common to find that, after two to four weeks, a self-expanding stent has expanded well into the wall of the artery, thereby supporting the vessel from within the smooth muscle layer, rather than from within the lumen. This is not a desirable result since most physicians intuitively feel it is advantageous to preserve the native, physiologically correct vessel properties as far as possible.
In addition, self-expanding stents are currently available only in 0.5 mm increments. This is a problem because exact sizing, within 0.1 to 0.2 mm expanded diameter, may be necessary to adequately reduce the effects of restenosis. Furthermore, these devices are often oversized by up to 30-50% to ensure location retention and vessel patency, thereby producing a chronic outward stress, as described above. Thus, greater selection and adaptability in expanded size is needed. As a result of these and other shortcomings, self-expanding stents have limited effectiveness in many important applications.
Heat expandable stents are similar in nature to self-expanding stents. However, this type of stent utilizes the application of heat to produce expansion of the stent structure. Stents of this type may be formed of a shape memory alloy, such as Nitinol. Still other types of heat expandable stents may be formed with a tin-coated, heat expandable coil. Heat expandable stents are often delivered to the affected area on a catheter capable of receiving a heated fluid. Heated saline or other fluid may be passed through the portion of the catheter on which the stent is located, thereby transferring heat to the stent and causing the stent to expand. However, heat expandable stents have not gained widespread popularity due to the complexity of the devices, unreliable expansion properties and difficulties in maintaining the stent in its expanded state. Still further, it has been found that the application of heat during stent deployment may damage the blood vessel.
In summary, although a wide variety of stents have been proposed over the years for maintaining the patency of a body lumen, none of the existing schemes has been capable of overcoming most or all of the above described shortcomings. As a result, clinicians are forced to weigh advantages against shortcomings when selecting a stent type to use in a particular application. Accordingly, an urgent need exists for a new and improved stent structure that successfully combines the desirable qualities of a balloon expandable stent and a self-expanding stent. It is desirable that such a stent be balloon expandable for providing accurate placement and sizing at a treatment site. It is also desirable that such a stent has sufficient radial strength to maintain patency of the lumen while subjected to substantial external forces. It is also desirable that such a stent be crush-recoverable, such that the stent returns to its deployed state in the event that the stent becomes crushed or pinched. It is also desirable that such a stent be provided with an effective constraining mechanism for holding the stent in the collapsed condition during delivery. It is also desirable that such a stent be configured to exhibit little or no longitudinal foreshortening during radial expansion. It is also desirable that such a stent be sufficiently flexible along the longitudinal axis to conform to the curved shape of a body lumen. It is also desirable that such a stent has the capability to conform to the interior of the body lumen. The present invention addresses these needs.